In radio frequency or wireless communication systems, a radio frequency (RF) receiver is a fundamental building block. Modern portable receiver design normally is implemented in a manner of directly down-conversion, where a low noise amplifier and a mixer are employed.
Designing a low noise amplifier and a mixer is difficult due to the multiple performance requirements, such as high linearity, large conversion gain, low noise figure, wider input match bandwidth, as well as low power consumption, etc. These requirements often conflict with each other, making a design more challenging.
A recently arisen design challenge in mobile platform design comes from an idea to remove SAW filter at front of a low noise amplifier for receiving GSM signals, and the main motivation is reducing cost. Other GSM users in the neighborhood of a user equipment at a distance about one meter can deliver as high as 1 mW RF power to the user equipment, i.e, 0 dBm at the low noise amplifier input port. To remove the SAW filter, the low noise amplifier must be able to handle this power level without suffering compression which will cause inter-modulation among wanted signals and unwanted block or interference signals.
Different types of low noise amplifiers exist, but they are typically not able to handle the above-mentioned interference levels, e.g. due to nonlinearities or too high voltage gain. Typically, extra high linearity is required for suppressing out-band block/interference signals to deal with the very strong interference level, such as 0 dBm at the RF input.
As an example, common source and common gate low noise amplifier structures cannot meet this requirement, because the interference signal swing is too high so that it creates nonlinearities at different nodes, such as input nonlinearity, interconnection nonlinearity or output nonlinearity.
Input nonlinearity can be improved by employing a differential low noise amplifier structure, and reducing the input impedance can reduce the input voltage swing. Feedback can also help to enlarge the linear region. However, output nonlinearity is difficult to deal with. To keep required linearity, the low noise amplifier output voltage swing must be low. Another difficult issue is the input match in the common source and common gate structure, as it has a very narrow bandwidth given acceptable input match. Adding a feedback path from the output to the input can improve the input matching, but it requires higher voltage gain at the output node. Otherwise, for the input matching the feedback resistance has to be small, leading to large contribution to noise figure due to the thermal noise of the feedback resistance. Increasing voltage gain of the low noise amplifier allows using a larger feedback resistance. Thus the noise contribution from the feedback resistance is reduced, but it directly contradicts with reducing the voltage swing at the output node to meet linearity requirement.
Common gate low noise amplifier has better linearity than common source and common gate low noise amplifier and good input matching for wide band, but the power gain is not enough. When connected to a passive mixer, the radio frequency front-end does not provide enough power conversion gain, and as a consequence other circuitries, like low pass filters, connected to the front-end will still contribute significant noise to the receiver chain. It is even worse for narrow band wireless communication like GSM, as the flicker noise in the low pass filter is usually very high. So basically, common gate low noise amplifier is also not suitable for the GSM case due to its high noise figure.